Summary The MD Helicopter Inc. 369D, serial number290459D, registrationC-GHFA, was operating from the operator's Horizon Camp near Adams Lake, 56nautical miles northeast of Kamloops, British Columbia, in heli-logging support functions. The pilot reported that at higher torque settings there was some resistence in the collective; the resistence subsequently went away. The main rotor blade track appeared to be out of alignment and the aircraft had a strong vibration. The pilot completed the crew and material moves then landed. Company maintenance personnel inspected the helicopter and found a crack in the lower skin of one of the five main rotor blades. Ce rapport est galement disponible en franais. Other Factual Information The operator reported that in the days leading up to the occurrence, the blades had been flying out of track1 for quite some time. Company personnel had to re-track them almost every shift, but the blades did not fly well together. Also during a flight, one of the riggers had noted that the skids were shaking excessively. The whole aircraft had a significant vertical vibration. The blades had been removed on the 18June2002 at 8350 flight hours when a 100-hour inspection was performed on the helicopter. A torque event2 inspection was performed 19.7hours and 197torque events before the 100-hour inspection. The blades were inspected twice on the morning of the occurrence: by the company aircraft maintenance engineer (AME) and during the pilot's pre-flight. The AME inspected the helicopter controls and main rotor blades at 8353.6total airframe hours (TAFH) and found the red blade cracked on the underside from the trailing edge to the spar. The failed main rotor blade P/N:500P2100-101, S/N:A340 had accumulated 2981.1total time since new (TTSN). The MD Helicopter369D is equipped with a five-blade, fully articulated main rotor, and the main rotor blades are colour-coded to facilitate balance and tracking functions. The 369DRotorcraft Flight Manual (RFM) states that for a sudden onset of vibration, set the helicopter down immediately and do not fly again until the source of the vibration has been determined. The MD Helicopter Inc. Maintenance ManualHMI-2, Chapter62-00-00, contains a warning at Paragraph2 - Main Rotor System Troubleshooting, which states in part, sudden onset of excessive and/or unusual main rotor vibration should be investigated immediately as to the cause, prior to continued flight. Under no circumstance should main rotor tracking be attempted to correct the problem until a thorough inspection of the main rotor blades, hub assembly and strap pack assembly has been performed. The five main rotor blades installed on the helicopter were a mix of MD Helicopter Inc. and Helicopter Technology Corporation (HTC) blades with various times in service. It is generally recognized in the industry that tracking and balancing certain combinations of blades can be difficult. HTC holds Supplemental Type Certificate (STC) numberSR09074RC, and in1999 began to manufacture all main rotor blades for MD Helicopters under a Parts Manufacturing Approval (PMA). An STC is a certificate issued when an applicant has received FAA approval to modify an aircraft from its original design. The STC, which incorporates by reference the related type certificate, approves not only the modification but how that modification affects the original design. The P/N: 500P2100-101 main rotor blades are essentially built the same as the blades originally built by MD Helicopters Inc. and the blades built by the preceding helicopter Type Certificate holder, McDonnell Douglas Helicopter Systems (MDHS). The blades were normally tied down when the helicopter was parked. About a week before the occurrence, the night before being flown to Adams Lake, the helicopter was parked at Campbell River with one rotor blade secured with a sock tie-down cord. The winds reached a high of eight knots that night. The MD Helicopter Maintenance Manual Chapter10-10-00, Parking and Mooring practices and instructions caution to take up slack but not to apply bending loads on blades when securing them with sock tie-down cords and to install blade socks on all blades. Bending loads are hazardous to the structural integrity of the blades. Normally, the blades become rigid in plane with centrifugal force exerted in rotation; when parked, the helicopter blades may be subjected to wind gusts that cause bending loads. Whenever severe storm conditions or wind velocities higher than 40knots are forecast, the helicopter should be hangared or evacuated to a safer area, or the blades must be removed as they can be subjected to severe bending loads. MDHS issued Service LetterSL369D-111, dated 11January1999, to establish a new approach to calculating retirement lives of various helicopter components. Components such as the main-rotor blades were to be assigned retirement lives based on information gathered from flight tests, fatigue tests, and field experience. Some operators reported exceeding the estimated amount of high-stress manoeuvres in the flight spectrum during daily operations, causing MDHS to re-evaluate the method of establishing limited lives which, until this time, only considered time in service (TIS). It was proposed that torque events be considered in the flight spectrum equation. Subsequently, on 15May2001, MDHI issued Service BulletinSB369D-201, annotated as mandatory compliance. On 24July2001, MDHI issuedSB369D-201R1. These bulletins contain criteria to assist operators in understanding the level of usage, the impact of that usage on the main rotor blade life, and the corresponding inspections required to find cracks that might occur. Under Canadian Aviation Regulations, service letters and bulletins themselves are not mandatory unless mandated by the foreign civil aviation authority, or referenced by an airworthiness directive (AD). MDHI publishes airworthiness limitations that establish life limits for helicopter components. In accordance with the MDHI Maintenance Manual, Chapter4, revision29, the life limit for the subject blade, part number500P2100-101, is 3530hours. The blades have an initial scheduled inspection interval of 100hours and, in accordance with SB369D-201R1, after a blade has accumulated 750flight hours and 13720torque events, operators are required to perform a main-rotor blade torque-event inspection every 35flight hours or at 200torque events whichever occurs first. No finite torque event number is established for the main rotor blades. The TSB previously investigated a main rotor blade failure (A01P0061) that was attributed to a manufacturing defect at the first lightening hole of the Cchannel at about station (STN) 36.5inches. During this previous investigation, MDHI studied 28blades to determine the mode of failure. An examination of the Cchannel was part of the study protocol. Of the 28blades, 4showed cracking at the first lightening hole in the Cchannel. Of the four blades that showed cracking, three came from one aircraft involved in operations with a very high number of torque events per hour. The fourth was used in a manner that reportedly exceeded the normal rotorcraft flight manual limits. The Cchannel cracks observed in this study did not appear to be from any manufacturing defect. Blades exposed to a high number of torque events were more likely to display cracks. The study showed that the cracked Cchannel found in the blade was rare but not unique. It also showed that if a crack were present, its progression would be discovered very early with the implementation of Service BulletinSB369D-201R1. Lightening holes are normally designed in a structural member to afford weight savings but may also affect its torsional, flexure or bending properties, and fatigue life. The weight saving for this main rotor blade is reportedly insignificant. Following the occurrence, the TSB Engineering branch was provided with 16main rotor blades to determine the frequency of occurrence of cracks at the Cchannel lightening holes.3 Four of the main rotor blades, sectioned and examined, were found to have cracks at the first lightening holes, about STN36.5. None of the blades with cracks in the Cchannel lightening holes had propagated to the outer skin (AppendixA - TSBLP069/2002). Given the small thickness of the Cchannel (0.28mm), any surface imperfection in the bore / lightening hole, would represent a large stress concentration factor. In at least two and possibly three cracks the initiation sites coincided with surface irregularities. The cracks were well developed meaning that they formed and propagated while the blades were well below the time limit of 3530hours. This points to a weakness in the blade and the insidious nature of the cracking (undetectable by conventional inspections). The structural integrity of the blade was compromised. There are no prescribed inspections for identifying lightening hole cracks, or any documented standards to accept material flaws in this part of the main rotor blades, like a crack in the Cchannel. A search of the Transport Canada and the Federal Aviation Administration (FAA) Service Difficulty Reporting (SDR) databases produced seven reports of main rotor blade cracking on the 369 series model helicopters, four of which were specifically identified and located about STN36.5. The main rotor blades for the MDHI 600Nmodel helicopter have a similar construction and are also subject to cracking at this location as revealed in two reports of the SDR databases. As part of the FAA requirement for certification of the main rotor blades, MDHI produced a stress analysis for Model369F helicopter4 based on a computer model to evaluate stresses at certain locations of the main rotor blade. Analysis of the stresses applied at blade STN36.5 were not determined. MDHI developed the inspection interval based on their analysis of crack growth. The incident main rotor blade S/NA340 was preliminarily examined at the TSB regional facility, and metallurgical testing was performed under TSB supervision by R.J.Waldron Company(1987) Ltd. who produced Technical Analysis Summary Report Number02-230. Transport Canada also provided the TSB with an example of a failed blade. It had about 2400hours in service and an estimated 17000TE. The lower skin was cracked and TSB engineering services confirmed that the crack located near blade STN36.5 originated from the Cchannel lightening hole. The visible crack on the exterior surface of the blade measured 4.5inches long and ran in a chord-wise direction from the approximate aft edge of the spar to the trailing edge of the lower skin at STN36.5. Once the blade was sectioned it was found that, in addition to the lower skin crack, the Vchannel at the trailing edge of the blade had a longitudinal crack that.measured 2.7inches long. This crack was roughly centred about the lower skin crack and the C channel crack, below the first lightening hole. No cracking of the spar or upper skin were detected in the area of the lower skin crack. (Photo1) The C channel crack fracture faces were found to exhibit beach markings and patches of deteriorated striations that are indicative of a fatigue mode of progressive cracking. The fatigue cracking was found to initiate from the inside diameter of the first lightening hole in the Cchannel of the blade. It progressed downward through the lower section of the channel and into the channel to skin adhesive. A line type fatigue initiation was observed on the inside surface of the lower skin below the Cchannel. The fatigue then progressed through the lower skin to its outside surface and spread both forward and rearward in a chord-wise direction from that point. Photo2. SEM image revealing pitting (101X magnification) The quality assurance process detailed by HTC for the method of inspection of the Cchannel consists of running a protected/covered finger along its length to check for surface flaws or imperfections, such as burrs or nicks. MD Helicopters Inc. has incorporated a finite number of torque events for the 600Nmodel helicopter which is based on retirement index number (RIN) accounting. The 369Series helicopter model is not subject to the same requirement.